1. Field of the Invention
The apparatus of the present invention relates generally to dial indicating bi-metallic thermometers and more particularly to a bi-metallic thermometer having a replaceable sensing and indicating element.
2. Description of the Prior Art
A bi-metallic thermometer comprises an enclosure consisting of a stem terminating in a case. A bi-metallic helical temperature sensing coil disposed within the stem imparts rotary motion to a shaft, which extends into the case and terminates in a pointer. The sensing coil, the shaft and the pointer rotate with variations in temperature, providing a visual indication on a graduated dial mounted in association with the pointer.
Although bi-metallic thermometers have found wide usage, historically such instruments have required replacement in a relatively short period of time. As a result of breakage or instrument failure, certain studies have discovered that the average life expectancy of a bi-metallic thermometer is approximately four years. Frequent replacement of such instruments has required the user to maintain a considerable inventory of replacement units or has necessitated prolonged shutdown of expensive equipment. Furthermore, even calibration of conventional bi-metallic thermometers requires removal from their operating environment, interferring with production processes. Similar interuptions occur when a thermometer must be replaced with a unit having a different temperature scale. In spite of these obvious problems, there has not been a successful bi-metallic thermometer whose replacement does not interfer with its operating environment.